crypto: drbg - use pragmas for disabling optimization

Replace the global -O0 compiler flag from the Makefile with GCC
pragmas to mark only the functions required to be compiled without
optimizations.

This patch also adds a comment describing the rationale for the
functions chosen to be compiled without optimizations.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>